page_title: Geological compass
section_title: Usage
hierarchical_section_title: Geological compass / Modern geological compasses / Usage
caption_attribution_description: English: Geological strike line and dip for a plane
context_page_description: There are a number of different magnetic compasses used by geologists to measure orientation of geological structures, as they map in the field, to analyse the geometry of bedding planes, joints, and/or metamorphic foliations and lineations. In this aspect the most common device used to date is the analogue compass.
context_section_description: At first sight it appears confusing to the novice user, for the numbers on the compass dial ascend in an anticlockwise direction. This is because the compass is used to determine dip and dip-direction of surfaces (foliations), and plunge and plunge-direction of lines (lineations). To use the compass one aligns the lid of the compass with the orientation of the surface to be measured (to obtain dip and dip direction), or the edge of the lid of the compass with the orientation of the line (to obtain plunge and plunge direction). The compass must be twisted so that the base of the compass becomes horizontal, as accomplished using the spirit level incorporated in it. The needle of the compass is then freed by using the side button, and allowed to spin until the damping action slows its movement, and then stabilises. The side button is released and the needle is then firmly held in place, allowing the user thereafter to conveniently read the orientation measured.  One first reads the scale that shows the angle subtended by the lid of the compass, and then depending on the colour shown (red or black) the end of the compass needle with the corresponding colour. Data are then recorded as (for example) 25°->333° (dip and dip-direction) or (plunge and plunge-direction).
  This compass has the most use by structural geologists, measuring foliation and lineation in metamorphic rocks, or faults and joints in mining areas.
caption: Strike line and dip of a plane describing attitude relative to a horizontal plane and a vertical plane perpendicular to the strike line
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